Field of the Invention
The invention relates to a method for channel estimation from received signals transmitted via a radio channel. The method can advantageously be used in mobile radio systems.
In a mobile radio system, information is transmitted from a transmitting radio station to a receiving radio station. This information arrives at the receiving radio station in the form of received signals. Due to various external influences, the received signals arrive at the receiving radio station via a number of propagation paths. The signal components which correspond to the various propagation paths arrive at the receiving radio station in the form of wave elements or partial waves at successive times. In the receiving radio station, there is now the problem of equalizing these signal components, which may further be influenced by other interference components, the problem of correcting the errors, and of decoding the transmitted information.
Parameters which take account of the channel conditions are determined within the receiving device, in order to evaluate the received signals. These parameters are, for example, channel impulse responses which are known, for example, from W. Koch, xe2x80x9cOptimum and Sub-Optimum Detection of Coded Data Disturbed by Time-Varying Intersymbol Interferencexe2x80x9d, IEEE Proceedings 1990, pages 1679-84, and are expressed by channel coefficients. These channel coefficients, which are used in a channel model, are used to superimpose different, successively arriving signal components of a received signal on one another in a suitable manner.
It is furthermore known to transfer the received signals to the baseband and to A/D convert them for obtaining digitized received signals. The digitized received signals as well as the channel impulse responses are supplied to a detector, which equalizes the received signals and carries out an error correction. The signal symbols reconstructed in the output of the detector are then decoded in a decoder, for example in a Viterbi decoder.
It is also known to use so-called training sequences for adjusting or equalizing receiving radio stations in mobile radio systems, see Mouly and Pautet, xe2x80x9cThe GSM System for Mobile Communicationsxe2x80x9d, 49. rue Louise Bruneau, F-91120 Palaiseau, France, 1992, pages 231-237. At predetermined times, the transmitting radio station transmits a sequence of digital signals which is known to the receiving radio station, that is to say whose data are available in undistorted form in the receiving radio station.
It is accordingly an object of the invention to provide a method for channel estimation which overcomes the above-mentioned disadvantages of the heretofore-known methods of this general type and which allows channel impulse responses to be determined in an improved manner, such that the determination of the channel impulse responses is resistant to interference sources.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for estimating a channel from received signals transmitted via the channel, the method comprises the steps of receiving, with receiving sensors of a receiving device, a plurality of received signals composed of at least one subscriber signal marked with transmitter-specific fine structures, the at least one subscriber signal being transmitted by a plurality of wave elements having differing incidence directions at the receiving sensors; obtaining, in the receiving device, an incidence direction of at least one of the wave elements; and determining directionally selective channel impulse responses from the plurality of received signals and the incidence direction of the at least one of the wave elements.
In the case of the method according to the invention for channel estimation from received signals transmitted via a radio channel, a receiving device is assigned a number Ka of receiving sensors, via which Ka received signals, which are assigned to the receiving sensors, are received. The received signals are composed of at least one subscriber signal which has a transmitter-specific fine structure impressed on it, in which case a k-th subscriber signal, k=1 . . . K, is transmitted through the use of Kd wave elements or partial waves whose incidence direction at the receiving point differs. In a first method step, at least one direction of incidence of a wave element of a subscriber signal is evaluated so that, in a second method step, directionally selective channel impulse responses are determined from the Ka received signals and the at least one direction of incidence.
In many applications, for example in radar, sonar or seismic measurement systems having only one transmitter or reflector, or having only a small number of transmitters or reflectorsxe2x80x94see e.g. U.S. Pat. No. 5,299,148 in this contextxe2x80x94the number Ka of receiving sensors is greater than the number Kd of wave elements to be evaluated per subscriber, so that, using the method according to the invention, Ka-Kd fewer channel impulse responses need be determined. This also results in a reduction in the complexity of the channel estimation.
In addition, the influence of interference sources is considerably reduced by concentrating of the incidence directions of the wave elements. The channel estimation becomes more accurate.
In accordance with a preferred embodiment of the invention, the directionally selective channel impulse responses are determined from training sequences of the subscriber signals, wherein the training sequences form the transmitter-specific fine structures. If the subscriber signals contain training sequences, which are known at the receiver end, more accurate channel estimation is possible than by using data which still have to be detected in the receiving device. Furthermore, the method can thus easily be implemented in existing mobile radio systems.
A further advantageous embodiment of the invention provides that subscriber signals from a plurality of transmitters or reflectors are superimposed and arrive as the received signals in the receiving direction, wherein these signals are transmitted at the same time in one frequency channel. In this case, the subscriber signals can be separated using a CDMA method (Code Division Multiple Access). Direction-related and subscriber-signal-related channel estimation thus allows the accuracy of the determined channel impulse responses to be improved in CDMA mobile radio systems as well.
In the simplest case, it is possible to use direction information from an xe2x80x9ca-priori-knowledgexe2x80x9d which is obtained, for example, from geometric/geographic considerations of the locations of the mobile stations and/or base stations, and/or of interference sources as well. No further calculation effort is thus required for direction estimation.
If the receiving device does not have a sufficient a-priori-knowledge, then it is advantageous to determine the incidence directions of the wave elements and/or of interference signals from the received signals. Continuously updated values for the incidence directions are thus available. This is of interest particularly for applications in mobile radio systems.
High-resolution direction-estimation methods are used to determine the incidence directions of the wave elements. Such high-resolution direction-estimation methods, such as the MUSIC (Multiple Signal Classification) or ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) methods, use knowledge of the complex radiation characteristics of the receiving sensors and/or specific geometric preconditions for the configuration of the receiving sensors in order to carry out an accurate direction estimation, which involves little signal processing complexity.
The resistance to interference sources is further improved by considering additional information relating to at least one incidence direction and/or a correlation matrix of interference signals in order to determine the incidence directions of the wave elements.
According to an advantageous embodiment of the invention, directionally not selective channel impulse responses, which are associated with the individual subscribers, are determined from the received signals, and the incidence direction of at least one wave element is determined from the directionally not selective channel impulse responses. The directionally not selective channel impulse responses which are determined for Ka received signals form good raw information for the direction estimation, since channel influences have already been considered. Furthermore, conventional channel estimation devices may be used for this directionally independent channel estimation.
If the method according to the invention is used in multiple-subscriber systems, then, for a subsequent evaluation, it is necessary to assign the determined channel impulse responses to transmitters or reflectors. The subscriber signals can be separated by individual training sequences in order to assign the directionally selective channel impulse responses to transmitters or reflectors. The training sequences are thus used not only for channel estimation but also for subscriber separation. Alternatively, the subscriber signals can be despread using individual subscriber codes in order to assign the directionally selective channel impulse responses to transmitters, by which the subscriber signals can be separated.
According to a further embodiment of the invention, the determined values are averaged over a time interval in order to determine the incidence directions of the wave elements. The incidence direction changes little within a time interval which may correspond to several times the coherence time of the channel impulse responses. Averaging improves the direction estimation, since random errors are reduced. If the data are transmitted in radio blocks, the averaging can be carried out for one radio block or else for a number of radio blocks. The number of radio blocks for averaging, that is to say the time interval, may in this case be varied, with changes in the incidence directions resulting in a change in the time interval. If the channel conditions change quickly, for example if the movement of a mobile station accelerates, then the direction estimation can be limited to a shorter time interval.
According to advantageous applications of the invention, relationships between the determination of the directionally selective channel impulse responses and data detection are used.
Thus, according to one embodiment, the K subscriber signals comprise sections carrying data and training sequences, in which case the directionally selective channel impulse responses are determined from the received signals resulting from the training sequences of the K subscriber signals, and the data are detected from the received signals resulting from those sections which carry data.
In this case, the determination of the directionally selective channel impulse responses and the data detection can be carried out from subscriber signals in a radio block. A channel estimation that is as current as possible is thus available for data detection.
As an alternative to this, the determination of the directionally selective channel impulse responses and the data detection can be carried out from subscriber signals in different radio blocks. This allows, for example, parallel processing to be carried out for a channel estimation and a data detection. It also allows the computation complexity or effort for a channel estimation to be reduced, by repeating the channel estimation only at relatively long intervals.
The incidence directions and/or the directionally selective channel impulse responses can be determined again, using a follow-up or tracking method, after a period which is longer than a frame structure related to a radio block, or information relating to the incidence directions and/or the directionally selective channel impulse responses can be permanently stored in the receiving device, provided such information is not time-dependent, or only very little dependent on time. An operation and maintenance center can advantageously initiate the updating of this information relating to the incidence directions, the directionally selective channel impulse responses and/or interference sources.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for a channel estimation from received signals transmitted via a radio channel, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.